The present invention relates to an improved method of forming a semiconductor device, and more particularly an improved method of forming doped regions through an epitaxial layer of a semiconductor device.
In semiconductor fabrication, it is often desirable to form doped regions which extend through an epitaxial layer, such as vertical channels or isolation regions. One common method of forming these doped regions is by using top-down diffusion. Top-down diffusion is a process which begins with the deposition of a full epitaxial layer on a surface of a substrate. After the full epitaxial layer has been deposited, a dopant is introduced to a selected location on top of the epitaxial layer. The wafer is then placed into a furnace, which causes the dopants to be driven down into the epitaxial layer. If the wafer is kept in the furnace for a sufficient time, the dopant will work its way through the entire epitaxial layer, forming the desired doped region.
There are three primary problems with the top-down diffusion process. First, in semiconductor fabrication it is often desirable for an oxide to be formed on top of the dopant and the epitaxial layer before placing the wafer into the furnace, because the oxide will protect the wafer and act as a diffusion barrier. The oxide may also be grown during the diffusion of the dopant. Unfortunately, in the case of boron dopant the oxide has the undesirable property that it will absorb a large percentage of certain dopants, rather than allowing all of the dopant to diffuse through the epitaxial layer. Therefore, when the oxide layer is formed immediately adjacent to the dopant, a large amount of dopant must originally be introduced to ensure that a sufficiently doped region is formed through the epitaxial layer. The second problem with this method is that the dopant must be diffused through the entire epitaxial layer. Since the diffusion time increases as the square of diffusion distance, the fabrication time could be decreased significantly if the diffusion distance were reduced. The third problem with this method is that the dopant diffuses both vertically and laterally and therefore creates a lateral diffusion region that is related to the thickness of the epitaxial layer. It is desirable to keep the diffusion region as small as possible in the lateral dimension to reduce device sizes. If smaller devices can be fabricated, chip sizes are reduced, which results in a greater number of chips that can be fabricated on a single wafer.
To solve some of these problems, another prior art method, top-down/bottom-up diffusion, has been used to create a doped region through an epitaxial layer. Top-down/bottom-up diffusion is similar to top-down diffusion, except that the dopant is introduced both before and after the deposition of the epitaxial layer. Temperature elevation then causes the dopants to diffuse both up and down through the epitaxial layer and meet in the middle. While this method does reduce the lateral dimensions of the doped region and decreases the time needed for the diffusion process, additional masking steps are necessary to form the additional doped areas. Since the time to form a single masking layer can be as much as one day, the time needed to form and remove the additional masking layers counteracts the benefits of the decreased diffusion time.
The present invention is an improved method of forming doped regions through an epitaxial layer during semiconductor fabrication. The method involves introducing dopants in the middle of the epitaxial layer and diffusing them bi-directionally to form the desired features. This is accomplished by depositing a first epitaxial half-layer on top of a substrate. Next, a dopant (or multiple dopants) is introduced on top of the first epitaxial half-layer. A second epitaxial half-layer is then deposited over the first epitaxial half-layer. The first and second epitaxial half-layers together form a full epitaxial layer of the desired thickness. After any other desired layers have been formed over the second epitaxial half-layer the wafer is heated to diffuse the dopants through the epitaxial half-layers. This method results in reduced total fabrication time, minimal lateral diffusion and a reduced amount of necessary dopants. The reduced total fabrication time also helps to minimize the rise of the buried layer.